1. Field of Invention
The present invention relates to a method of, and apparatus for, reducing d.c. offset in a signal, in particular in a signal in a radio receiver such as an FM or Digital Audio Broadcasting (DAB) radio receiver.
The invention is of particular relevance to the reduction of d.c. offset in a signal resulting from the downconversion of a received radio signal from a carrier frequency to an intermediate frequency, and more especially to a signal resulting from the direct downconversion of a received radio signal from a carrier frequency to baseband frequency.
2. Description of Related Art
It is well known in the art to downconvert received FM radio signals at a carrier frequency to a lower, intermediate frequency (IF) before detection and other processing takes place. This has the advantage that the detection and other processing stages need only be designed to operate at a relatively narrow range of frequencies around the intermediate frequency.
Similarly, it is known to downconvert DAB radio signals at a carrier frequency to a lower, intermediate frequency (IF) before detection and other processing takes place. In the final stages of processing, the IF signals are downconverted again to the baseband frequency.
Alternatively, it is known to downconvert DAB radio signals directly from the carrier frequency to the baseband frequency without first downconverting to an intermediate frequency. In that case, the radio signals at the carrier frequency are mixed with a signal from a local oscillator (LO) at the receiver operating at the carrier frequency. However, a significant problem with direct downconversion of DAB radio signals is that large d.c. offsets are often introduced into the downconverted signal, for instance because of interference from the local oscillator feeding back into the signal received at the antenna of the receiver, or feeding back into other parts of the receiver circuitry.
It is known that attempts have been made to reduce the d.c. offsets in downconverted signals by including feedback circuits which operate by comparing a downconverted signal to a constant reference signal representing the expected d.c. level of the downconverted signal and then altering the d.c. level of the downconverted signal in dependence upon the comparison. However, the use of such feedback circuits can be problematic as the expected d.c. level may depend upon the characteristics of components of the receiver circuitry, such as amplifiers, which may vary from component to component and which may change over time. Accordingly the expected d.c. level may differ from or drift away from the constant reference signal, causing additional d.c. offset problems.